If you go on You Tube you’ll get a video covering this topic on the channel Knowledgia. It was the first video I watched about the topic and it gave me great insights into the hows and whys of that whole ordeal.
France and Poland signed a treaty of friendship in 1921 after the Polish-Soviet war which stated that if one or the other were the victim of an unprovoked attack, the other would help.
In May 1939 Poland and France signed a new treaty binding the militaries of the two nations, but this treaty only named Germany as a potential aggressor, thus France was technically not obligated to declare war on the Soviet Union after the Soviets invaded Poland.
The treaty signed between Poland and the United Kingdom however referred to “hostilities with a European power”.
Britain and Poland then also signed a treaty in August 1939 promising mutual military assistance if either were attacked by a European country. This means Britain was technically obligated to go to war with the USSR under the terms of the August 1939 treaty.
However, this was signed by the British hoping to discourage German expansionist ideals.
Thus, a secret clause referred to an attack by Germany specifically.
However if the aggressor was a different European national then Britain and Poland would only to consult together on measures to be taken in common.
Still, under the terms of the pact Britain was legally obligated to declare war on the Soviet Union.
When the Soviet Union invaded the Polish ambassador to London did request assistance.
Lord Halifax said the following: “As regards Soviet aggression we were free to take our own decision and to decide whether to declare war on the USSR or not.”
Still, the British used the secure clause of the 1939 pact to justify not going to war with the Soviet Union saying that they only guaranteed Poland’s protection in case of German aggression.
But fun fact, the Polish was not aware of this secret clause. The British did consider making it public to explain their lack of action to the Poles, but they decided not to. The British also said in October 1939 that the Poles should have understood that the pact only covered Poland in case of German aggression.
British ambassador to Moscow wrote in a secret telegram on 18 September 1939
“I do not myself see what advantage a war with the Soviet Union would be to us…our war aims are not incompatible with reasonable settlement (in Poland) on ethnographic and cultural lines.”
There was also the fact that neither Britain nor France were really prepared for war. They declared war on Germany but I mean neither of them really did much of anything, hence the Phoney War.
Seeing this the USSR thought that the Allies wouldn’t make another empty declaration of war as with Germany, so they thought it was safe to invade Poland without repercussions.
Plus Germany and the Soviet Union were historic rivals, so many people in Britain and France thought that maybe they’ll pick a fight with each other later on.
They also knew about the non-aggression pact between Germany and the Soviet Union, but they figured it was not a military alliance and if they declared war on the Soviet Union it might result in a German-Soviet alliance.
Should this happen Germany would have the natural resources from the Soviet Union as well as their own large industry.
Pre-war the focus was mainly on Germany and their expansion. Neither Britain nor France cared much about the Soviet Union. Britain and France weren’t ready for a war with even Germany, so they didn’t want to add the Soviet Union to the list of enemies.
So France wasn’t technically obligated to declare war on the Soviet Union, but Britain could’ve legally declared war. But in the end the British did not.
Britain and France wanted to stop Germany. They didn’t necessarily want to protect Poland specifically, just stop Germany. They had no designs on the Soviet Union.
Their treaties were to stop German aggression, not Soviet aggression. One can even argue they didn’t care that much about Polish independence as long as Germany stopped its expansion.